Salmonellagenomic island 3 is an integrative and conjugative element and contributes to copper and arsenic resistance ofSalmonella enterica

Salmonellagenomic island 3 (SGI3) was first described as a chromosomal island inSalmonella4,[5],12:i:-, a monophasic variant ofSalmonella entericasubsp.entericaserovar Typhimurium. The SGI3 DNA sequence detected fromSalmonella4,[5],12:i:-isolated in Japan was identical to that of a previously reported one across entire length of 81 kb. SGI3 consists of 86 open reading frames, including a copper homeostasis and silver resistance island (CHASRI) and an arsenic resistance operon in addition to genes related to conjugative transfer and DNA replication or partitioning, suggesting that the island is a mobile genetic element. We successfully selected transconjugants that acquired SGI3 after filter mating experiments using theS. entericaserovars Typhimurium, Heidelberg, Hadar, Newport, Cerro, and Thompson as recipients. Southern blot analysis using I-CeuI-digested genomic DNA demonstrated that SGI3 was integrated into a chromosomal fragment of the transconjugants. PCR and sequencing analysis demonstrated that SGI3 was inserted into the 3′ end of the tRNA genespheVorpheR. The length of the target site was 52 or 55 bp, and a 55-bpattIsequence indicating generation of the circular form of SGI3 was also detected. The transconjugants had a higher MIC against CuSO4compared with the recipient strains under anaerobic conditions. Resistance was defined by thecusgene cluster in the CHASRI. The transconjugants also had distinctly higher MICs against Na2HAsO4compared with recipient strains under aerobic conditions. These findings clearly demonstrate that SGI3 is an integrative and conjugative element and contributes to the copper and arsenic resistance ofS. enterica.